1. Field of the Invention
This invention relates generally to a system and method for the drilling, completion and work-over of oil and/or gas wells. Specifically, the invention relates to the control of oil and/or gas wells during the period when the blow-out preventer (BOP) is closed, or is in the process of being closed, due to events, such as kicks, that occur during drilling, completion, or while working over the well.
2. Description of the Related Art
During the drilling of subterranean wells, a fluid (“mud”) is typically circulated through a fluid circulation system comprising a drilling rig and fluid treating equipment located substantially at or near the surface of the well (i.e., earth surface for an on-shore well and water surface for an off-shore well). The fluid is pumped by a fluid pump through the interior passage of a drill string, through a drill bit and back to the surface through the annulus between the well bore and the drill pipe.
A primary function of the fluid is to maintain a primary barrier inside the well bore to prevent formation fluids from entering the well bore and flowing to surface. A blow-out preventer (BOP), which has a series of valves that may be selectively closed, provides a secondary barrier to prevent formation fluids from flowing uncontrolled to surface. To achieve a primary barrier inside the well bore using the fluid, the hydrostatic pressure of the fluid is maintained at a level higher than the formation fluid pressure (“pore pressure”). Weighting agents may be added to the fluid to increase the fluid density, thereby ensuring that the hydrostatic pressure is always above the pore pressure. If, during drilling of the well bore, a zone is encountered having a higher pore pressure than the fluid pressure inside the well bore, an influx of formation fluid will be introduced into the well bore. Such occurrence is an undesirable event and is known as taking a “kick.” This same situation can occur not only during drilling, but also during completion, work-over or intervention.
When a kick is taken, the invading formation liquid and/or gas may “cut,” or decrease, the density of the fluid in the well bore annulus, such that an increasing amount of formation fluid enters the well bore. Under such circumstances, control of the well bore may be lost due to breach of the primary barrier. Such an occurrence may be noted at the drilling rig in the form of: (1) a change in pressure in the well bore annulus, (2) a change in fluid density, and/or (3) again in fluid volume in the fluid system tanks (“pit volume”). When a kick is detected, or suspected to have entered the well bore, fluid circulation is conventionally halted and the well bore closed in/shut in by closing the BOP. The pressure buildup in the well bore annulus, pit gain and shut in drill pipe and casing pressures are then monitored and measured. Appropriate well-killing calculations may also be performed while the well is closed in. Before resuming operations, a known well-killing procedure may be followed to circulate the kick out of the well bore, circulate an appropriately weighed fluid (“kill fluid”) into the well bore, and ensure that well control has been safely regained. Typically, the intent of the operator while circulating a kick out of a well and circulating the kill fluid is to ensure that another kick does not enter the well. If, however, while performing these tasks another kick enters the well, the entire well bore condition again changes. The operator may subsequently lose control of the well, because the monitored and measured parameters are transient and confusing as a result of the previous kick. Furthermore, it will be more difficult to ensure that the well control procedures were successfully completed and that the operator has effectively regained control of the well bore to permit recommencement of operations.
One of the requirements for safely and effectively killing the well, and circulating an appropriate kill fluid, is to hold the pressure inside the well bore as constant as possible, above the formation pore pressure and below the formation fracture pressure. The first task is, therefore, to ensure accurate knowledge of the pore and fracture pressures as a function of depth, and to properly calculate the correct fluid weight to be circulated. If the pressure inside the well bore oscillates too much during the circulation of the kick out of the well bore, then there is high risk that the pressure inside the well bore will fall below the formation pressure and a secondary kick will be taken while the process of controlling the first one is ongoing. Alternatively, if the pressure inside the well bore oscillates and reaches the fracture pressure, fluid losses into the formation are induced. This causes the integrity of the well bore to be severely jeopardized and makes the necessary well control operations much more difficult. As previously stated, such scenarios should be avoided.
The two most common methods for circulating the kill fluid and circulating the kick out of the well bore are: the Driller's method and the Wait and Weight method. The Driller's method may be utilized when kill weight fluid is not yet available for circulation. In the Driller's method, the original fluid weight may be used to circulate the influx of formation fluids from the well bore. Thereafter, kill weight mud (“KWM”) may be circulated into the drill pipe and the well bore. Although two circulations may be required to effectuate the Driller's method, this method may be quicker than the subsequently described variation. In the Wait and Weight or “Engineer's” method, KWM is prepared and then circulated down the drill string and into the well bore to remove the influx of formation fluids from the well bore and to kill the well, in one circulation. This method may be preferable in order to maintain the lowest casing pressure while circulating the kick from the well bore, thereby minimizing the risk of damaging the casing, fracturing the formation and/or creating an underground blow-out. In either the Driller's method or the Wait and Weight method, a substantially constant pressure inside the well bore, above the pore pressure and below the fracture pressure, should be maintained.
The Driller's method and the Wait and Weight method are only suitable, however, for use in commonly encountered well control situations. There are several other more complex situations faced while regaining control of the well bore which require a more sophisticated approach. In situations where the drill bit is off bottom, there is no drill string inside the well bore, or the drill string is parted, more complex methods are needed, such as volumetric, dynamic volumetric, or lube and bleed methods, to ensure that control of the well is restored. In some cases, there is no margin to allow circulation of the influx without fracturing the formation. In such cases, the alternative is to bullhead the influx back into the formation and not to circulate the influx out of the well bore. These complex methods are more difficult to implement because several variables must be controlled, and this complexity is often more than the rig crew can handle. Thus, well control experts are frequently moved to the rig site to assist with well control, if these more complex well control methods are employed.
In the conventional drilling of a well, the blow-out preventer (BOP) remains open and the return of the fluids from the well is directed through a fluid return line to a shale shaker and fluid system tanks on the surface. Thus, the well is drilled while being open to the atmosphere and without the possibility of applying pressure at surface. If an indication of an influx is detected at anytime, the BOP is closed and a well control procedure is initiated. When a fluid influx occurs it is a sign that the pressure inside the well bore is lower than the formation pressure, and that the fluid weight should be increased to restore a balanced condition. As previously described, there are many different ways of controlling the well after the detection of a fluid influx. The preferred way in which a well is controlled is dependent on a number of factors including, but not limited to, the configuration of the well, the operational condition of the well at the time the detected influx, whether the drill bit is on bottom or off bottom, whether the drill string is parted, and/or whether the drill string is completely out of the well. The Driller's method and the Wait and Weight method, described above, are two of the most popular ways to control a well after influx detection when the drill bit is on bottom, however, other methods and variations thereof are implemented depending on the particular drilling company. When the BOP is closed, the return of the fluid is diverted to the rig well control choke manifold through a choke line, wherein one or more adjustable chokes control the pressure (i.e., backpressure) in the choke line and in the annulus.
Conventional well control procedure involves several steps, which are well known to those skilled in the art:
First, the well is shut in by closing the BOP in order to measure the pressures in the annulus and inside the drill string, and thereby provide an indication of the amount of additional pressure required to rebalance the well;
Next, the fluid influx is circulated out of the well while controlling the well pressure at the surface appropriately to prevent a second influx from entering the well bore (as previously stated, in some cases there is no margin to allow circulation of the influx without fracturing the formation, which leads to the decision to bullhead the influx back into the formation instead of circulating it out of the well bore);
Next, a heavier fluid is circulated through the well to restore the hydrostatically overbalanced condition, which is a required condition for many oil and/or gas well drilling operations;
Finally, confirmation is made that the well is hydrostatically overbalanced by checking the pressures in the annulus and inside the drill string so that the BOP can be reopened to resume operations.
During execution of the conventional well control procedure, the steps are conducted while relying on pressure readings as measured in the injection line, called standpipe pressure and as measured in the choke line, called casing pressure, and in a few cases, on the volume of fluid in the pits. Relying solely on pressure readings, however, does not allow the driller to completely understand downhole events, such as ascertaining the hydrostatically underbalanced condition based on the time the influx was taken, verifying that an influx indeed entered the well bore or ensuring that the well is under control. Furthermore, using the pit volume as indicator of well condition during a well control method is far from accurate.
In addition to well control, the BOP may be closed for other reasons, such as to conduct a leak-off test in order to determine the fracture pressure of the formation. Current systems and methods for determining formation fracture pressure and formation pore pressure, however, are inaccurate. For example, the pore pressure derived from stabilized surface standpipe and casing pressure readings measured after the BOP has been closed is often far from accurate, and in many cases, there is no influx into the well bore. The sole reliance on pressure readings and their misinterpretation leads to this result. Moreover, the use of inaccurately measured fracture and pore pressures can have serious consequences for the economics of the well. For instance, the pore pressure is used to define the new mud/fluid weight required to be circulated through the well after a kick is detected in order to return the well to a hydrostatically overbalanced condition. Thus, if the determined pore pressure is inaccurate due to a lighter fluid presence in the well bore, and not the result of a hydrostatically or dynamically underbalanced situation, the typical procedure is to needlessly introduce heavier weight fluid into the well bore.
As stated, the misinterpretation of non-kick events, based solely on pressure readings or pit volume measurements, can lead to false alarms of kicks. An action that may be taken in response to these false alarms is the circulation of fluid with an unnecessary increase in fluid weight, which can cause subsequent operational problems, such as a loss of circulation, a stuck pipe and/or a low rate of well bore penetration. For instance, the fluid weight used to kill the well is selected to be much higher than needed, thereby causing severe problems when operations are resumed. In certain situations, this results in the well being prematurely abandoned. Even if the well is not abandoned, the huge amount of resources wasted by the lack of accuracy and controllability of current well control methods is costly.
Furthermore, the misinterpretation of downhole events can, in many cases, lead to the taking of secondary influxes while attempting to control the first kick. This can and often does lead to well blow-outs. For example, there were 28 out-of-control blow-outs alone in the United States in 2008. Brian Kraus, DRILLING CONTRACTOR, July/August 2009, at 100-01. Most of these blow-outs caused property damage, some caused environmental damage, and at least one blow-out caused a busy highway to be diverted because the fire at the drilling site was too close. Another reason that many kicks can get out of control and turn into devastating blow-outs is the lack of experience and knowledge of the personnel at the rig site concerning such events. In many instances, the on-site personnel are unable to interpret the fluid influx situation, perform the necessary calculations, and/or properly implement the required well control procedures.
Improving the safety and controllability of well control operations after the BOP has been closed is a major concern on the majority of worldwide drilling rigs. In an attempt to improve well control procedures and the overall safety of conventional operations, several systems and methods have recently been developed which focus on improved kick detection, while others concentrate on controlling pressures more accurately during circulation of the kick and displacement of the kill mud. Most of these systems and methods, however, rely solely on pressure monitoring and measurement to regain control of the well after the BOP has been closed. While pressure measurements can, in some limited cases, provide a good indication of the events inside the well bore with the BOP closed, pressure measurements alone do not provide a full and complete understanding of what events are occurring downhole. Likewise, pressure measurements alone do not ensure that false indications of kicks are prevented or permit the accurate assessment of fracture and pore pressures. Considering the problems associated with current strategies of well control when the BOP is closed, an improved well control system and method provides several advantages. This application is based on U.S. provisional patent application No. 61/311,166, filed on Mar. 5, 2010, which is incorporated herein by reference.
3. Identification of Objects of the Invention
An object of the invention is to accomplish one or more of the following:
Provide a system and method to permit the safe cessation of drilling operations in response to an indicated or suspected onset of a kick event;
Provide a system and method for controlling oil and/or gas wells after closing the blow-out preventer;
Provide a system and method for more accurately determining the fracture and pore pressures of the formation;
Provide a system and method for confirming if the fluid weight is insufficient to hydrostatically balance exposed formations, and if confirmed, determining an accurate value for the fluid weight increase required to restore hydrostatic balance or overbalance;
Provide a system and method for controlling the pressure at any specific, selected depth inside the well bore between specified limits, such as between the formation fracture pressure and the formation pore pressure;
Provide a system and method for maintaining control of oil and/or gas wells such that drilling and other operations on these wells may be conducted in sensitive formations;
Provide a system and method which reduces the risk of well blow-outs, which could result in life and/or properties losses;
Provide a system and method for enhancing hands-on training and competence assessment using the well control equipment of the rig;
Provide a system and method for controlling an oil and/or gas well such that experts not located at the rig site may be involved earlier in well control procedures; and
Provide a system and method for the collection, interpretation and display of well control-related data for timely and effective participation in well control procedures by experts located remotely from the rig.
Other objects, features, and advantages of the invention will be apparent from the following specification and drawings to one skilled in the art.